Down-hole reciprocating pumps are commonly used for moving fluid from the bottom of a well bore up to the ground surface, as is well known in the art.
In general, a conventional pump of this kind comprises a tubular reciprocating piston slidably received within a tubular pump barrel. The piston is attached at its upper end to the lower end of the rod string, for reciprocation thereby. The barrel seats at its upper end in a seating nipple forming part of the wellbore tubing string. The piston has a close fit within the barrel, so that it effectively seals against the barrel wall and functions to lift fluid above it on the up stroke. A travelling check valve is positioned in the bore of the piston, commonly at its lower end, and functions to control the ingress of fluid into the piston bore. A standing check valve is positioned at the lower end of the pump barrel bore and functions to control the ingress of fluid into the barrel bore beneath the piston. These valves are commonly one-way check valves with a ball and seat structure.
In operation, when the piston is on the down stroke, the ball of the travelling check valve unseats and the hydrostatic head, arising from the column of fluid in the tubing, keeps the ball of the standing check valve seated. Thus, the travelling check valve is open and the standing check valve is closed on the down stroke. As the piston descends, fluid in the pump barrel bore moves through the travelling check valve, into the piston bore, and on up into the tubing bore. When the piston reaches the bottom of its down stroke and reverses into the up stroke, the ball of the travelling check valve seats, thereby trapping the fluid standing above it. Thus, as the piston rises, fluid above the travelling check valve is lifted upwardly. At the same time, the low pressure created by the upward stroke of the piston causes the ball of the standing valve to unseat and oil is drawn in from the well bore and reservoir. Fluid from the well bore thus follows the rising piston by passing through the standing check valve and entering the barrel bore. This stroking sequence is repeated again and again to gradually lift a fluid column to ground surface.
In wells where gas or vapour are produced along with liquid, the pump can be subject to poor efficiency due to gas lock. Gas lock occurs when gas is trapped between the standing and travelling check valves. On the down stroke, the hydrostatic weight of fluid above the pump tends to keep the travelling check valve closed. The gas trapped in the pump barrel compresses as the piston descends, but does not achieve enough pressure to open the travelling check valve. (If there were incompressible liquid alone in the barrel bore, the pressure would rise sufficiently to unseat the travelling check valve ball, as previously stated.) The trapped gas does, however, create enough pressure in the barrel bore to hold the standing check valve closed, so new fluid may not enter the pump. The end result of the foregoing is that the gas-locked pump fails to move fluid. In addition, heat build-up due to the lack of lubricating fluid may damage the check valves.
A number of solutions to eliminate gas lock have been tried in the prior art, each entailing a number of disadvantages.
One solution is to physically "tap down" the pump by lowering the piston to the bottom of the barrel bore. This rattles the check valves, so that the travelling check valve ball is unseated and gas can escape upwardly. However, this can damage pump components. Also, it is difficult to tap the pump, as the above-ground pumping unit must be disconnected and adjusted to cause the piston to over-travel and tap the barrel.
Another solution is shown in U.S. Pat. No. 4,867,242 to Hart. This patent involves an assembly adapted to automatically trip the travelling check valve open as the piston approaches the bottom of the down stroke. An unseating rod is positioned above the standing check valve and is adapted to protrude into the travelling check valve to unseat the ball near the bottom of the down stroke. However, striking the ball with the unseating rod again and again damages the sealing surface of the ball, reducing the life of the travelling check valve. Also, the unseating rod restricts the flow passage to such an extent that the flow capacity of the pump is significantly reduced. As well, this arrangement cannot be fitted to standard pumps.
Finally, the inventors are aware that competitors have publically tried another solution, involving automatically tripping the travelling check valve at the beginning of each down stroke. It involves a port sub carried by the travelling check valve with a push-rod held against its ball. A sliding keep sleeve allows the piston sleeve and port sub to be fully extended on the up stroke or fully compressed on the down stroke. While this device may be fitted to a standard pump, it still suffers the disadvantages of the rod damaging the ball and significantly reducing the flow capacity of the pump.
Thus, there remains a need for modification of a down-hole reciprocating pump which successfully deals with the problem of gas lock without reducing the pumping capacity of the pump.